1. Field of the Invention
The present invention is generally related to computer display systems, and particularly to hardware and software systems that display images in a plurality of colors, that display data from multiple software application programs, or that combine several images on cathode ray tubes (CRT) or other like displays commonly used in computer and data processing systems. The present invention more particularly relates to the display of graphics and character output in color, or in multiple shades of gray, and to the display of data where animation or movement of one or more displayed objects is desired.
2. Description of Related Art
A graphics display system encodes data representing the graphic or character information to be displayed into discrete picture elements or pels. A digital memory, typically a random access memory (RAM), stores the encoded picture elements (pels) which make up a display frame. The graphic display is then generated by a video processor which accesses the stored pel data, decodes the data into signals required for the display monitor to generate color or multigray shade images, and transmits the signals to the CRT or other display monitor. The pels are rapidly displayed in a raster scan of the display monitor faceplate. The scan, typically horizontal, is comprises of a number of scan lines each comprised of a number of pels. The pel data stored in the memory is typically encoded in one of two forms. In bit-encoded graphics systems each pel is represented by several, e.g. three, binary units or bits of data. The bits are organized into planes with each plane having one bit for each pel on the display monitor. Thus, the three bits of data representing a particular pel are stored at the same vertical and horizontal offset in three separate bit planes. The final video display screen is generated by simultaneously accessing the bit planes, passing the resulting set of three bits to a translation table which generates the control signals required to create the color or shade of gray. The individual bit planes contain only part of the information necessary to create the final display structure, it is only through the combination of the three planes and decoding of the associated colors or shades of gray, that the display image structure is realized.
A second method of encoding is lateral bit encoding. In this method, a sequential set of bits is used to encode each pel of the display image. For example, a sequence of two bits may be used to encode each pel on the display. If one bit is used to encode each pel that bit simply indicates whether the pel is to be on or off. If on, the system specified foreground color is displayed, otherwise the background color is displayed. If two bits per pel are used, the non-zero value indicates which one of three foreground colors to display. A lateral bit encoded image can be stored in a single bit plane and is accessed sequentially in relative screen positions, for example, from left to right, and top to bottom.
The prior art contains several examples of both bit plane and lateral bit encoded graphics display systems.
U.S. Pat. No. 4,691,295 to Erwin et al., discloses a graphics system that employs four bit planes for bit encoded graphics display. Erwin et al. allow use of the bit planes as a group to form a single bit encoded image, and allows selective display of data from individual bit planes. However, Erwin et al. do not suggest a display system that can operate in either bit encoded or lateral bit encoded modes to create systems with distinctly different "personalities".
Other devices have used multiple memory buffers to store and display a series of lateral bit encoded images on the screen. U.S. Pat. No. 4,653,020 to Cheselka et al., commonly assigned, discloses a system where multiple buffers are used to store encoded characters generated from multiple applications. Each of the buffers is displayed in a separate window on the display screen. There is no merging of data from the several buffers. Cheselka et al. are concerned only with character displays and not with the display of graphics images.
U.S. Pat. No. 4,317,114 to Walker discloses a display processor where several lateral bit encoded image planes are overlayed and merged with data from a host computer system to create the final display screen image. Walker doesn't provide multiple use of the buffers and fails to teach a method for controlling image mixing.
Iwami, in U.S. Pat. No. 4,682,297, commonly assigned, provides a graphics display system that creates a composite image by merging multiple images from separate memory buffers. The images are merged based on a selection of a "transparent" color which allows the background image to be viewed wherever that "transparent" color exists. This implementation is useful for creating displays with moving objects since the moving object can be "moved" (i.e. erased and redrawn) in a single plane which is then merged with other planes containing non-moving objects Iwami, however, provides an apparatus for merging only two image buffers. It cannot be readily extended to three or more buffers, and doesn't teach the dual use of buffers.
Thus, the prior art display systems typically support only one of the two image encoding methods, or primarily support one method with the second method receiving only limited support. This functional rigidity limits the application of a particular graphics display system and is a significant disadvantage, particularly in the general purpose display system field.